1. Field of the Invention
The present invention relates to a process comprising producing electric energy, steam and carbon dioxide in concentrated form from a hydrocarbon feedstock. The invention further comprises optionally producing synthesis gas based products combined with the process.
2. Description of the Related Art
Electric energy is produced in a combined cycle power plant integrated with a reforming plant where a gas turbine is fueled by hydrogen containing gas. (Integrated Reforming Combined Cycle (IRCC)). A major problem in such a process is operating the gas turbine at conditions having minimum nitrogen oxide emission and simultaneously achieving optimal electric energy and steam production.
A process for producing electric power, steam and concentrated carbon dioxide is published on Internet as an article entitled xe2x80x9cNEW TECHNOLOGY can cut carbon dioxide emissionsxe2x80x9d. In this publication there is described a process comprising reacting natural gas with steam subsequently resulting in a hydrogen containing gas which is combusted in a combined cycle gas turbine producing electric power.
From Japanese patent application JP608041, it is further known to apply a hydrogen fired turbine for production of electric energy. According to this application, natural gas and oxygen in a mol ratio 1:0.5 to 1:0.7 is reacted by partially oxidizing the fuel to generate hydrogen and carbon monoxide. Air is supplied to a pressure swing absorption oxygen separator (PSA) and the oxygen is then delivered to an autothermal reactor (ATR) where the natural gas is transformed to hydrogen and carbon monoxide. The reformed gas enters a shift reactor in which the carbon monoxide is converted to carbon dioxide. The gas mixture is then introduced into a membrane separator in which hydrogen is separated from the carbon dioxide. The separated CO2 is washed-out and desorbed later on. The hydrogen substantially free from carbon compounds is used in a gas turbine for generating electric power. This process requires oxygen, demanding a power consuming PSA unit. According to the application flow sheet, the natural gas must be decompressed nearly to ambient pressure to permit the addition of oxygen. After the PSA separation, the oxygen must be compressed a second time. All of these extra compressions reduce the efficiency of the process.
The main object of the present invention is to provide an improved process for generating power using steam reforming of a hydrocarbon feedstock, in which a substantial part of the generated CO2 is separated as a highly concentrated CO2 gas stream and where the emission of nitrogen oxides is within acceptable levels for conventional gas turbines.
Another object of the present invention is to utilize at least a part of the formed synthesis gas of the power generating process for the production of synthesis gas products, especially ammonia, methanol and/or dimethylether.
With regard to electric energy generation, the present process will compete with conventional power plants based on the combustion of hydrocarbon feedstock, such as natural gas. However, one major disadvantage of simply combusting hydrocarbons is that the emission of carbon dioxide as the exhaust from the combustion only contains minor amounts of carbon dioxide which at present cannot be economically recovered. The emission of nitrogen oxides (NOX) which varies depending on the operating conditions may also constitute an emission problem.
A major problem when reducing the emission of carbon dioxide and NOX is obtaining the desired emission reduction without an unacceptable reduction of efficiency of the process with regard to power generation. The first step in evaluating the basic process in view of the above requirements was the synthesis gas production step. Having considered various methods, the inventors found that an ATR would give several advantages and it was decided to investigate further the best way of running the ATR. Contrary to what was taught by the above Japanese patent application, it was found that the ATR should be an air driven reactor, i.e., not an oxygen driven reactor. The application of the ATR seemed to offer several advantages in terms of degrees of freedom. Thus, the operating pressure could be chosen in view of the overall economy of the concept. The methane slip could be varied in view of the operation of downstream units and finally, the synthesis gas produced in the ATR would be a relatively lean gas suitable for the gas driven turbine and comparable to fuel mixtures being used in proven, large scale combined cycle plants (IRCC).
Useful hydrocarbon feedstock for such a process will be natural gas, nafta, various petroleum distillates, etc. By applying a pre-reformer ahead of the ATR, the flexibility with regard to feedstock will be fairly great. However, the preferred feedstock will be natural gas.
The NOX problem was found to be strongly related to operating conditions of the gas turbine. The NOX-formation is correlated to the flame temperature in this turbine. Accordingly, provisions for regulating the flame temperature should be made. The range of gas mixture to be combusted in the turbine could be selected through the design of the process in order to keep the flame temperature at a desired level and still maintain acceptable power generation. The flame temperature in the turbine is largely determined by the composition of the fuel gas. It was found that an airdriven ATR would provide a lean hydrogen based fuel gas mixture compatible with gases being used in IGCC-plants. It was found advantageous to extract process air for the ATR at the discharge of the gas turbine""s air compressor and boost to the required ATR-injection pressure. Further, the air flow could be adjusted to meet the agreeable level of methane slip, and the composition of the fuel-gas mixture could be compatible with an acceptable level of NOX-formation in the gas turbine combustion system. The nitrogen extracted with the air from the gas turbine is returned to the turbine part as a component of the fuel gas mixture, thus largely maintaining the turbine mass flow.
If need be, moderate steam injection can be applied to reduce the NOX-formation in the turbine, and an optimal design of the burner can also reduce the NOX-emission.
One alternative within the concept of the invention is to combine the ATR with a reformer exchanger. It was found that this option could increase the recovery of CO2 in concentrated form.
In order to obtain maximum flexibility, the basic power generating concept could be combined with the production of various products based on the existing process streams. Thus, a methanol unit could utilize some of the synthesis gas from the ATR and an ammonia plant could utilize some of the hydrogen/nitrogen gas separated from the carbon dioxide subsequent to the shift reaction of the synthesis gas. The only extra units required for the ammonia plant would be a conventional membrane separation unit and a methanator upstream of the ammonia synthesis reactor.
The scope of the invention comprises forming synthesis gas in an air driven ATR unit, heat exchanging the formed synthesis gas and thereby producing steam. At least part of the cooled synthesis gas is then treated in a CO-shift reactor, which may be one single unit or two CO-shift reactors, one low temperature reactor and one high temperature reactor. The gas stream is further treated in a carbon dioxide unit for formation of a concentrated stream of carbon dioxide and one stream is a lean hydrogen containing gas which is at least partly combusted in a combined cycle gas turbine for the production of electric energy. Air from the turbine is supplied to the ATR unit. The exhaust from the gas turbine is heat exchanged for the production of steam which together with steam generated upstream is utilized in a steam turbine for production of electric energy.
The ATR unit can be combined with a reformer exchanger and the feedstock can be split between these two units. Preferably 50-80% of the feedstock is fed to the ATR. A pre-reformer can be arranged upstream of the ATR unit. A minor part of the steam generated in the process can be fed to the gas turbine for diluting the hydrogen containing gas and thereby lowering the flame temperature in the gas turbine. At least part of the exhaust from the gas turbine can be recycled to the ATR as an oxygen source or combined with the air supply to the gas turbine. Part of the synthesis gas can be utilized for methanol production and this production can be performed in various ways as described above in connection with FIG. 1.
Part of the gas from the carbon dioxide separation unit can be utilized for ammonia production. In this case, stream is fed to a membrane separation unit for separating out hydrogen which is mixed with another hydrogen containing gas stream, whereby the mixed stream will have a nitrogen:hydrogen ratio of 1:3. The nitrogen from the membrane unit is returned to the main hydrogen containing gas stream subsequently fed to the gas turbine.